The invention relates to a getter trap for removing hydrogen and oxygen from liquid metal, such as sodium, and, more particularly, to such a getter trap for removing hydrogen and oxygen impurities from the liquid metal coolant of liquid metal nuclear reactors.
Liquid metal nuclear reactor systems typically include primary and secondary liquid metal coolant loops. The primary liquid metal coolant, such as sodium, is heated as it passes through the nuclear core of the liquid metal nuclear reactor, and is subsequently cooled by indirect heat exchange with the secondary liquid metal coolant in a heat exchanger. Liquid metal circulating in the secondary coolant loop then passes through a second heat exchanger wherein the thermal energy from the secondary liquid metal coolant is used to heat water flowing in a third loop to produce steam, which, in turn, is used to drive an electricity generating device, such as an electrical generator.
The secondary liquid metal coolant often contains undesired hydrogen and oxygen. The primary system also contains oxygen and hydrogen from the types of sources such as oxygen-bearing impurity gases in the argon or other cover gas, and the "dirt burden" on the surface of materials of construction. Hydrogen, such as that produced by corrosion in the third loop, passes into the secondary liquid metal coolant through the second heat exchanger. A typical hydrogen load in a secondary liquid sodium coolant loop in a 300 Megawatt power liquid metal nuclear reactor is 5432 kilograms of hydrogen per 30 years. Oxygen impurities are produced at a lesser rate than hydrogen impurities are produced. Thus, the secondary coolant loop often includes a means to eliminate the undesired hydrogen and oxygen from the secondary liquid metal coolant.
One method of removing the hydrogen and oxygen impurities from liquid metal is the use of a cold trap. Cold trapping action depends upon the decreasing solubility of impurities with decreasing temperature. In a typical cold trap, a liquid metal system is cooled as it passes through a subsidiary system which includes a vessel where the impurities are precipitated and held as solid phases.
In a conventional cold trap, the liquid metal enters the top of the trap from a heat exchanger and flows downwardly through the outer annulus, or "downcomer". The flow direction reverses at the bottom of a cylindrical volume packed with mesh. The liquid metal then flows upwardly through the cylinder and out through the end of the cylinder to an exit pipe. In general, nucleation of precipitate, and subsequent growth thereof, occur largely on the coldest surfaces of the cold trap. Typically, the coldest sections of cold traps are the bottom of the cold trap, the outer vessel wall, and the extended surface of the inlet section of mesh. The nucleation of precipitate and subsequent growth of the nuclei throughout these areas of the cold trap eventually restrict the flow path of the liquid metal through the cold trap and the cold trap is plugged even though only a small portion of the cold trap is filled with impurities. I disclosed a cold trap in my U.S. Pat. No. 4,291,865, which issued Sept. 29, 1981 to the assignee of the subject invention having a radial design for removing impurities, such as oxygen, from a liquid metal.
U.S. Pat. No. 3,853,700 discloses a trap for carbon in liquid sodium that utilizes binary alloys of iron and from 0.5 to 30 weight percent of titanium, from 0.5 to 25 percent vanadium, or from 0.5 to 5 percent manganese.
U.S. Pat. No. 3,993,453 relates to a getter trap of a composite with a substrate of a metal having a large coefficient of thermal expansion, such as nickel or nickel alloy, having a coating, which fractures upon heating, of zirconium or zirconium alloy for a nuclear fuel element.
Getter traps have also been used to absorb hydrogen from liquid alkali metals. U.S Pat. No. 3,622,303 relates to such a getter comprised of a barrier of a first layer of iron, nickel, tantalum, columbium and their alloys, and a second layer of palladium, platinum and their alloys.
U.S. Pat. No. 4,312,669 describes a gettering alloy for gases which include oxygen and hydrogen that is selected from the group consisting of an alloy of zirconium, vanadium and iron, whose composition, in weight percent, when plotted on a ternary diagram, lies within a triangle having as its corners the points defined by:
(a) 75 percent zirconium, 20 percent vanadium and 5 percent iron; PA1 (b) 45 percent zirconium, 20 percent vanadium and 35 percent iron; PA1 (c) 45 percent zirconium, 50 percent vanadium and 5 percent iron. PA1 (a) 75 percent zirconium, 20 percent vanadium and 5 percent iron; PA1 (b) 45 percent zirconium, 20 percent vanadium and 35 percent iron; and PA1 (c) 45 percent zirconium, 50 percent vanadium and 5 percent iron.
The disclosure of U.S. Patent No. 4,312,669 is incorporated by reference herein.